The Burma Circle of the Geological Survey of India and their Contributions to the Geology of Myanmar

By THAN HTUN (GEOSCIENCE MYANMAR)

EPISODE:35
Geology and Ore Deposits of the Bawdwin Mines
This episode is the continuation of the previous Episode 34.

History of the Mines
In 1910-11, the permanent way of the railroad was at last in good order. The vertical shaft was enlarged and equipped with a surface plant. It has two hoisting compartments, a ladder, and a pump way. Winding and steam plants were provided for the inclined shaft, which was ventilated by two fans driven by a waterwheel. The total development work for the year amounted to over 2,000 feet, and the winning of ore in the upper workings to provide a silicious flux for the smelters was extended. 2,687 ½ tonnes of ore were mined, the average value of which was approximately 19.38 per cent lead, and 7.72 ounces of silver per tonne were railed to the smelters at Mandalay.
In 1911-12, the smelters were removed from Mandalay to Nam Tu. The slags and ores smelted at Mandalay totalled 12,054 ½ tonnes, and at Nam Tu, 15,774 ½ exclusive fluxes. A lead refinery was erected at Nam Tu. The railway line from Malpwe to Bawdwin was re-sleepered and ballasted, resulting in a material decrease in maintenance charges. Several Italians were engaged in mining work. The total development work done in the mine was 3,624 lineal feet and 8,159 cubic feet. The tonnage mined was 5,568 tonnes, of which a large portion was low-grade iron ore, which was only useful for fluxing. The company took over a prospecting license issued for the old Chinese mines at Mo-ho-chaung, about 35 miles from Bawdwin.
In 1912-13, the Chinaman lode was opened up on the 171’ level. Total developmental work equalled 6,127 lineal feet and 9,051 cubic feet, 6,222 tonnes of ore were mined, and 32,019 tonnes of slag were recovered by sluicing in the Bawdwin Valley. The 3rd level, at 300 feet in the vertical shaft, was opened up, and an adit level 1,860 feet long was finished to carry off the drainage from the No 1 level, which was formerly pumped through the shaft. The total tonnage of slag and ore treated at the smelter was 37,254 tonnes, containing approximately 80,000 ounces of silver, the increase in silver being due to the larger proportion of ore now smelted with the slag.
In 1913-14, ending 30 June 1914, the finances of the company were recognized, and the Burma Mines Company was taken over by the “Burma Corporation Limited”. Great strides were made in opening up the mine both in the vertical shaft and Dead Chinaman Tunnel sections.
The development work for the year was 15,199 lineal feet, exclusive of diamond drilling and 18,910 cubic feet. The total tonnage of slag, ores, and fluxes treated in the smelters amounted to 51,482 tonnes, yielding 6,872 tonnes of lead, containing approximately 175,906 ounces of silver. The small furnace at Mo-ho-chaung reached an output of five tonnes per day for part of the period.
In the year 1914-15, the Chinaman ore body was opened up further, and the mine prepared for stopping in the upper levels. The Tiger Tunnel was driven further and should be connected to the Chinaman ore body by the end of 1916. Steps have been taken to smelt mixtures of roasted ore and concentrate estimated to produce about 1,500 tonnes of lead and about 140,000 ounces of silver per month.

General Geology
Topography
Bawdwin district contains two types of sharply contrasted relief. West of Nam Tu, the surface is mountainous and entirely occupied by steep slopes that are produced by the erosion of many streams. Flat ground, either as plateaux or in valley bottoms, is conspicuously absent, the nearest approach to it being the sinuous narrow tracts of alluvium which rarely border the banks of the larger streams. Even the more extensive valleys are deep and V-shaped, separated from one another by steep, knife-edged ridges, their sides scored by innumerable glens and ravines that make up the amazing network of the feeder drainage system of the country. Dense, temperate evergreen forest generally clothes the hills from base to summit. The higher features range from 5,500 to 6,500 feet above the level of the sea. East of the Nam Tu and extending a few miles to the west of the river in the Mong Tat neighbourhood, the true Shan plateau type of country is found, though here again, the usual flatness is diversified by the deep valleys of the Nam Tu and some of its smaller tributaries. The change from one type of scenery to the other is very abrupt and follows the boundary of the Plateau Limestone.

Stratigraphy
General
The Bawdwin ore deposits are enclosed in a series of ancient rhyolites and rhyolitic tuffs, which form a dome-shaped structure protruding through the younger Pangyun strata. Going west, an overthrust band of Nam Hsim sandstones is crossed before the rocks of the Chaung Magyi series are reached. These form an ancient land surface and, with their associated intrusive granites, build up the highlands of Tawngpeng. The younger Palaeozoic rocks found east of Bawdwin and are especially well exposed in the gorge of the Nam Pangyun were deposited in succession on the old land surface. The following rock groups have been met within the Bawdwin district: –

The Tawnpeng Granite.
The Northern or small map intrusive granite of Tawnpeng forms most of the higher country west of Bawdwin. The rock is an ordinary white granite consisting of quartz, orthoclase, microcline, and biotite. It usually bears evidence of intense crushing. La Touche’s statement that it contains no tourmaline is incorrect, as later work has shown that tourmaline does occur in it, though quite sporadically.

The Chaung Magyi Series.
This is the oldest sedimentary formation of the district and occupies a wide extent of the country in the western area, coming between the Tawnpeng granite on the west and the Palaeozoic rocks on the east. Its inner border with granite is very irregular, owing to the intrusion of a large tongue in the latter neighbourhood of Loi Ma-raw. The series consists of soft, well-foliated, slaty shales, phyllites, and greywackes with subordinate sandy horizons, and although it forms much of the highest land, it yields exceedingly few good exposures. The Chaung Magyi rocks to the north of Bawdwin are of slightly different facies, and sandy layers are perhaps commoner.

Bawdwin Rhyolite Series.
The elongated irregular area occupied by these rocks has been carefully mapped and found to comprise a narrow strip of country running in a general northwest and southeast direction. From Tiger Camp to the point in the upper Stern Valley where they finally disappear beneath the Pangyuns is not more than three miles, in a straight line. The series consists of true rhyolites, rhyolitic tuffs, true tuffs of various kinds, an occasional band of volcanic breccia, and subordinate layers of sandstone and felspathic grit. I regard the whole exposure as a kind of low dome rising from underneath the Pangyun beds which surround it. The main axis of the dome runs north-west-south-east. It is not always easy to separate the base of the Pangyuns from the uppermost beds of the Bawdwin Rhyolite series, for the Pangyun conglomerate is always succeeded below by shales and grits-certainly of no great thickness but still almost identical with those above it. Many of these lower Pangyun felspathic grits contain much tuffaceous matter, and again, the uppermost tuffs of the Bawdwin series are often gritty.

Pangyun Beds.
In an unpublished manuscript report written in 1906, Dr J M Maclaren described under the title of “Banyan Beds” a series of “fairly thin-bedded rocks made up of red and white sandstones chocolate micaceous shales and quartzites with subordinate dark shaly beds and occasional conglomerates” occurring near the Bawdwin mines. He was able to prove that they were younger than the rhyolites and the series to which he gave the name of the Bawdwin beds. Maclaren also described an unfossiliferous series below the Nyaungkangyi beds to the west of Lopah in the valley of the Nam Pangyun, “at first sight”, he writes, “they would appear to underlie the latter, but they are so very much fresher and younger in lithological appearance that the final impression left in the mind of that they are much younger than the Nyaungkangyi beds.” He regarded the two sedimentary series, the Bawdwin and Banyan beds, as being very closely connected-the latter overlying the rhyolites unconformably. Regarding their age, he writes, “The sedimentary members probably occupy a horizon in the Ordovician or maybe Upper Silurian. The solution to the question is not practicable from the single section run by the writer.”

Naungkangyi Beds.
The Pangyun series passes up conformably into the Nyaungkangyi beds of the Ordovician age, and there is some difficulty in finding a horizon to map as their junction. I have taken it as the top of a thick series of sandy purple mudstones that underlies the last Nyaungkangyi fossiliferous shale. In stream-beds where continuous sections are met, this horizon is not so difficult to find, but in other situations, the line taken is more or less arbitrary, for exposures are few and far between. As the map shows, the Nyaungkangyi band enters from the south and strikes the north. It is about 1 ½ miles broad at first. It becomes wider in the Pangyun Valley, gradually thinning out towards the north-north-west before it finally disappears below the overlying Plateau Limestone in the Nam Krak Valley. In most of the places where I have crossed this band, I have succeeded in finding characteristic Nyaungkangyi fossils; cystidean plates and Rafinesquina sp. are the most common.

Panghsapye Graptolite Bed.
Immediately above the purple sandy shales at the top of the upper Naungkangi beds, a thin band of carbonaceous shales containing large numbers of graptolites is found. I have found the southern extension of the band where it crosses the bed of the Nam Tu, about a mile below its junction with the Nam Pangyun and the mainstream, where it is a band of black slate containing abundant specimens of Monograptus sp.

Nam Hsim Sandstones.
La Touche’s general summary of the rocks of this group is as follows: –
A sandy series at least 2,000 feet thick, sometimes very coarse in texture, and elsewhere fine-grained and compact, hard and splintery.

Occasionally, their boundary with the lower rocks is marked by beds of coarse conglomerate, which consist of waterworn pebbles and boulders of Chaung Magyi quartzites.

Plateau Limestone.
To the east of the Nam Hsim zone, the Plateau Limestones occupy the whole country except where they are overlain in the southeast of the map by the red beds of the Namyau series of the Jurassic Age.

Namyau Beds.
The Namyau series of Jurassic Age rests unconformably on the Plateau Limestone. It is distinguished by the dark-red to purplish-red colour of its sandstones, though beds of grey and pepper and salt sandstones and bands of yellow clay are sometimes interstratified in the higher parts of the series.

Recent and Sub-Recent.
In addition to the recent alluvium, which borders some of the larger streams in the district, I have noticed raised river terraces in both the Nam Kung and Nam Pangyun streams, which points to the recent uplifts in the region.

References:
Brown Coggin J, 1917: Geology and Ore Deposits of the Bawdwin Mines, Records of the Geological Survey of India, Vol. XLVIII, Part 3.

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